DESCRIPTION OF THE PRIOR ART
Data transmission systems are known wherein all the subscriber stations are connected in series in a loop circuit transmitting signals in a given direction of rotation (for example, CH patent No. 550 521). Systems of this type are subject to the disadvantage that an interruption at any point in this loop circuit renders it impossible to transmit data over the entire loop.
Data transmission systems of this type are also known wherein an interruption at a particular point in the loop circuits only affects data transmission in one part of the loop. In all these systems: all the subscriber points and a plurality of switching stations are connected in series in the loop circuit, parallel line sections disposed in parallel with the loop line are each connected with two switching points and each switching point contains a switch controlled by a failure detector and adapted for switching between a loop circuit section and a parallel line section. As a result of this arrangement, only the subscriber stations disposed between the respective two switching points in the loop circuit are excluded from data transmission because of the failure, whereas all the other subscriber stations in the remaining part of the loop circuit and the parallel line section which have been connected in, are capable of transmitting data.
In a known system of this type (CH patent No. 551 118), the parallel line sections are each disposed between two immediately consecutive switching points, thus forming an auxiliary loop circuit in which the direction of the signals is opposite to that of the loop line. The disadvantage of this system is that, through the inclusion of the auxiliary loop circuit, when a failure occurs, the transmission path is increased by a distance of almost the entire length of the loop line. This causes transit-time errors - particularly at high bit-rates. To obviate failures produced thereby, signal regenerators must be connected into the auxiliary ring line. If the loop line is simultaneously interrupted at two points, not only the affected loop line sections but also the part of the ring line disposed therebetween will be by-passed and the subscriber stations located in this part rendered ineffective. A failure in a single switching point can render the entire data transmission system ineffective.
In the case of another known system of a different type (US Pat. Nos. 3 458 661 and 3 519 935), the loop line is designed as a bifilar loop connected to a central point. Each switching station is disposed in the forward and return line of the loop. When a failure occurs, the forward and return lines of the loop are connected together. As a result, the entire portion of the bifilar loop line located on the side of the resulting connection which is remote from the central station, is short-circuited and, accordingly, all the subscriber stations disposed in this portion are excluded from data transmission. In the case of this system it is also possible for a failure in one of the switching points to render the entire transmission system ineffective.